The present invention relates to methods to make strong, tough, and lightweight whisker-reinforced glass-ceramic composites. The method can involve forming a self-toughening structure generated by viscous reaction sintering of a complex mixture of oxides. The whisker-reinforced glass-ceramic preferably is strong, tough, and/or lightweight. The present invention further relates to strong, tough, and lightweight glass-ceramic composites used as proppants and for other uses including, but not limited to, armor plating, electronic, optical, high-temperature structural materials and applications, as a low dielectric constant substrate material in high-performance packaging applications; or window materials for the mid-infrared range.
The use of certain inorganic whiskers and fibers to reinforce glasses, glass ceramics, and ceramics has been known and practiced. Whiskers are typically characterized as relatively short, single-crystal fibers of small diameter, typically less than 100 microns. Fibers on the other hand can be multicrystalline or amorphous and are long enough to be used in woven or other types of interlocking networks, filter tows or fabric. Whiskers are typically incorporated in a selected glass or ceramic matrix as a randomly dispersed phase.
Fibers are more commonly used in an oriented or interlocking alignment. Load transfer by the matrix to the fibers through shear is the means by which fibers strengthen glass or ceramic bodies. The load transfers stress from the glass or ceramic matrix to the relatively long and high modulus fibers. The fibers can also impede crack initiation and propagation through the matrix material.
Whiskers can provide strengthening in a similar manner, but load transfer to the whiskers by the matrix is more limited because of the limited length and aspect ratio of the whiskers. Because whiskers are relatively short, they cannot carry as much load compared to the longer fibers. It is more difficult to take full advantage of the intrinsic strength of whiskers compared to fibers for this reason. Whisker reinforcement in ceramic and glass-ceramic materials is often used to increase toughness. A toughened ceramic material improves crack resistance, increases fatigue lifetime and/or provides a noncatastrophic mode of failure. Non-catastrophic failure is highly desirable in applications where repair can be facilitated and information about failure conditions is important.
Silicon carbide, silicon nitride, alumina, and carbon whiskers have all been used to reinforce non-metallic matrices. For example, U.S. Pat. No. 4,324,843 describes SiC fiber reinforced glass-ceramic composite bodies where the glass-ceramic matrix is an aluminosilicate composition. U.S. Pat. No. 4,464,192 describes whisker-reinforced glass-ceramic composites of an aluminosilicate composition. U.S. Pat. No. 4,464,475 describes similarly reinforced glass-ceramics with barium osumilite as the predominant crystal phase.
The use of whiskers in ceramic composites can improve the fracture toughness of the ceramic composite because of the whiskers' ability to absorb cracking energy. The whiskers appear to toughen the composites by deflecting crack propagation, bridging cracks and by whisker “pull-out.” Whisker “pull-out” occurs when the ceramic matrix at the whisker-matrix interface cracks. When a crack-front propagates into the composite, many of the whiskers can bridge the crack line and extend into the ceramic matrix surrounding the crack. For the crack to grow or propagate through the ceramic, these whiskers must be either broken or pulled out of the matrix. As these whiskers are pulled out of the matrix, they provide a bridging force across the faces of the crack, reducing the intensity of the stress at the crack tip. In this way, the whiskers absorb the energy that would propagate the crack. Whisker pull-out reduces the tendency of a composite to crack and also inhibits crack propagation. U.S. Pat. Nos. 4,543,345; 4,569,886; and 4,657,877 relate to silicon carbide whisker-reinforced ceramic composites.
The production of glass-ceramic composites with whisker or fiber reinforcement usually involves dispersion of the whiskers or fibers in a green body prior to firing or sintering the green body to produce the final glass-ceramic reinforced composite. The methods in U.S. Pat. Nos. 4,543,345; 4,569,886; and 4,657,877 recite preformed whiskers dispersed in a ceramic precursor prior to forming a green body for sintering. Processes involving dispersion of preformed whiskers in a green body material have been difficult to successfully implement because whiskers have a tendency to agglomerate resulting in non-uniform concentrations of whiskers throughout the green body and ultimately in the ceramic composite. Non-uniform whisker concentration results in significant variance in the extent of reinforcement and toughening. As the percent by weight of whiskers in a green body material increases, agglomeration and clumping of whiskers increases. In addition, powdered ceramic precursor material may become imbedded within clumped whiskers. After sintering, the presence of these powders can significantly weaken the whiskers' reinforcing abilities.
A variety of granular particles are widely used as propping agents to maintain permeability in oil and gas formations. Three grades of proppants are typically employed: sand, resin-coated sand and ceramic proppants. Conventional proppants exhibit exceptional crush strength but also extreme density. A typical density of ceramic proppants exceeds 100 pounds per cubic foot. Proppants are materials pumped into oil or gas wells at extreme pressure in a carrier solution (typically brine) during the hydrofracturing process. Once the pumping-induced pressure is removed, proppants “prop” open fractures in the rock formation and thus preclude the fracture from closing. As a result, the amount of formation surface area exposed to the well bore is increased, enhancing recovery rates. Proppants also add mechanical strength to the formation and thus help maintain flow rates over time. Proppants are principally used in gas wells, but do find applications in oil wells.
Relevant quality parameters include: particle density (low density is desirable), crush strength and hardness, particle size (value depends on formation type), particle size distribution (tight distributions are desirable), particle shape (spherical shape is desired), pore size (value depends on formation type and particle size, generally smaller is better), pore size distribution (tight distributions are desirable), surface smoothness, corrosion resistance, temperature stability, and hydrophilicity (hydro-neutral to phobic is desired). Lighter specific gravity proppants can be desirable, which are easier to transport in the fracturing fluid and therefore can be carried farther into the fracture before settling out and which can yield a wider propped fracture than higher specific gravity proppants.
Proppants used in the oil and gas industry are often sand and man-made ceramics. Sand is low cost and light weight, but low strength; man-made ceramics, mainly bauxite-based ceramics or mullite based ceramics are much stronger than sand, but heavier. Ceramic proppants dominate sand and resin-coated sand on the critical dimensions of crush strength and hardness. They offer some benefit in terms of maximum achievable particle size, corrosion and temperature capability. Extensive theoretical modeling and practical case experience suggest that conventional ceramic proppants offer compelling benefits relative to sand or resin-coated sand for most formations. Ceramic-driven flow rate and recovery improvements of 20% or more relative to conventional sand solutions are not uncommon.
Ceramic proppants were initially developed for use in deep wells (e.g., those deeper than 7,500 feet) where sand's crush strength is inadequate. In an attempt to expand their addressable market, ceramic proppant manufacturers have introduced products focused on wells of intermediate depth.
Resin-coated sands offer a number of advantages relative to conventional sand. First, resin coated sand exhibits higher crush strength than uncoated sand given that resin-coating disperses load stresses over a wider area. Second, resin-coated sands are “tacky” and thus exhibit reduced “proppant flow-back” relative to conventional sand proppants (e.g. the proppant stays in the formation better). Third, resin coating typically increases sphericity and roundness thereby reducing flow resistance through the proppant pack.
Ceramics are typically employed in wells of intermediate to deep depth. Shallow wells typically employ sand or no proppant.